Such a method is known, for example, from EP 0 925800 B1. In that document, a magnetically coupled sensor circuit is used in an alternating current circuit for temporarily switching off a residual current circuit breaker. In this method, a direct current source is connected to the circuit protected by the residual current circuit breaker. Furthermore, a direct saturation current is supplied from the direct current source into the circuit protected by the residual current circuit breaker in order to magnetically saturate the sensor circuit of the residual circuit breaker and in this way interrupt the residual current circuit breaker so that it is temporarily out of operation and in this way permits a test in the circuit.
Other methods for determining the loop resistance of a power supply network are known from documents DE-AS 20 35 274, DE 40 13 490 A1 and DD 274 681 A1.
Loop resistance measurements are used to make statements about the resistance values occurring in the network. The loop resistance is determined by the external conductor or phase, hereon called L1 conductor, and the ground or protective earth conductor, hereon also called PE conductor. Resistance values that are too high may prevent the triggering of the fault current circuit breaker, hereon called FI, that is provided in the power supply network, and thus may pose a danger to humans and animals. As a result of a too high loop resistance and the current flowing in the fault case, high voltages may be released from the network lines in a fault case and may expose the equipment cases or the body of humans or animals to a dangerous contact voltage. In particular in critical facilities, such as hospitals, the contact voltage must not exceed 25 V.
When discussing the loop resistance, two common network forms must be differentiated, the so-called TN network and the so-called TT network.
In the TN network, the star point of the generator is directly grounded. The connected devices are connected with the star point via the protective earth conductor. The connection is made via the so-called PEN conductor related to the TN-C network; i.e., together, via the neutral and protective earth conductor or via a separate neutral conductor N and the protective earth conductor PE (in the so-called TN-S network). Often, a combination of both networks exists. In this case, it is called a TN-C-S network. By building the network, a body contact always becomes a short circuit. The fault current is therefore relatively high, and in many cases fuses or circuit breakers can be used as protective devices for switching off a faulty device. These other protective measures are generally described by using the term grounding. In the case of such networks, it can be assumed that the loop resistance is very small and is essentially determined by the internal resistance of the network. The loop resistance is obtained from the resistance of the L1 conductor and the PE conductor as well as the PEN conductor.
The loop resistance of the real TN network is in particular composed of the generator internal resistance, the internal resistances of the transformers, the line resistances of the high voltage-low voltage conductors, and the transition and line resistances of the electric installation of the final consumer or inside a building.
A value of 2 Ohms is considered sufficient for the total grounding resistance of all system earths. This makes it possible that in the case of a ground fault of an external conductor, the voltage of the protective earth PE or of the PEN conductor do not have unacceptably high values in relation to the earth potential. In the case of floors with low conductivity, the entire earth resistance of the supply network may have a value of up to 5 Ohms if it can be assumed that in the case of a ground fault in an external conductor the earth transition resistance also has a correspondingly high value.
In the TT network, the start point of the generator is also directly grounded. The devices connected to the network are connected to ground connections that are independent from the grounding of the generator. When the network is built, a body contact becomes a ground fault. In the case of poor grounding, this may lead to high loop resistances, which may make a fault current circuit breaker ineffective. Especially in such a case, a loop resistance meter or a process for operating such a system would be reasonable, since they would allow a calculation of the maximum current flowing in case of a ground fault. From this, it could be deduced whether a preceding fault current circuit breaker or fuse would be triggered when the contact voltage is kept. By determining the loop resistance, a statement can be made about the quality of the network, and in particular about the respective grounding conditions. If the ground resistance is supposed to be analyzed in more detail, a special ground meter is necessary. The loop resistance is composed of the resistance of the L1 conductor and of the PE conductor as well as the ground resistance.
The loop resistance of the real TT network is composed of the generator internal resistance, the internal resistance of the transformers, the line resistances of the high-voltage and low-voltage conductors, the transition and line resistances of the electric installation at the consumer or in the building, and the resistance of the earth.
The previously addressed fault current circuit breaker is responsible for switching off all poles of operated devices within 0.2 seconds if, due to an isolation fault, a dangerous contact voltage occurs. Since the actual switch-off times of such circuit breakers are significantly shorter, fault current circuit breakers offer an especially effective protection.
The acceptable maximum values for the grounding resistance in the TT network depend on the acceptable contact voltage and the nominal fault current of the fault current circuit breaker. Depending on the grounding condition, this resistance may be high. In the TN network, the switching-off is always ensured by the low impedance. Since human protection is safely ensured with fault current circuit breakers, they are increasingly used in both of the previously mentioned networks.
All conductors (conductors L1, L2, L3 and N) leading from the network to the protected devices are passed through the cumulative current transformer. Since in a fault-free state the sum of the currents flowing in and out is zero, the magnetic alternating fields of the conductors cancel each other. In this case, no voltage is induced in the output winding of the cumulative current transformer.
In the case of a ground fault of a conductor, or in the case of a body contact of an operated device, a partial current flows via the earth to the current generator and back. Because of this, the sum of the input and output currents is no longer zero. For this reason, a voltage that actuates an electromagnetic trigger is induced in the output winding of the cumulative current transformer. This trigger switches all poles of the network. A fault can be simulated with a test key. This makes it possible to test the trigger function of the FI circuit breaker, but not the grounding of the system to be protected.
The switch-off time of a fault current circuit breaker must not exceed 200 ms, if--in the case of alternating fault currents--the nominal fault current, and--in the case of pulsating direct fault currents--the 1.4-fold nominal fault current is flowing. If these currents are exceeded by a factor of 5, fault current circuit breakers must reliably switch off within 40 ms. As a result, the measurement of the loop resistance must take place within a short time if high test voltages are used.
Known methods for determining the loop resistance of a power supply network of the initially mentioned type in which the internal resistance is determined according to the differential quotient of voltages and currents provide for energizing the loop of L1 conductor and PE conductor with a current load. As a result, the existing fault current circuit breaker is triggered by the fault current generated in the PE conductor. Previously known measuring methods and instruments load the L1-PE loop with an approximately 10 A peak current during a half-wave. This current regularly causes the triggering of the fault current circuit breaker.